Musnad Ahmad 32:1128

Yazid ibn Harun narrated to us, Humayd informed us, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) injured his foot, so he sat in a loft of his, the stairs of which were made of palm trunks, and he swore an oath to abstain from his wives for a month. His companions came to him to visit him, so he led them in prayer while sitting, while they were standing. When the time for the next prayer arrived, he said to them: "Follow your Imam; if he prays standing, then pray standing, and if he prays sitting, then pray sitting with him." He said: He descended after twenty-nine days. They said: "O Messenger of Allah, you swore an oath for a month." He said: "The month is twenty-nine [days]."